It is known in the art to manufacture flexible substrates by applying a polyimide precursor resin solution directly onto a conductor, followed by drying and curing, as disclosed, for example, in JP-A 59-232455, JP-A 61-275325, JP-A 62-212140, and JP-A 7-57540. Another method of applying a polyimide precursor resin solution in several divided portions onto a conductor is disclosed, for example, in JP-A 2-180682, JP-A 2-180679, JP-A 1-245586 and JP-A 2-122697.
However, the method of applying a polyimide precursor resin solution onto a conductor has the problem that unless the ultimate polyimide layer on the flexible substrate has a thickness of at least 20 microns, the substrate is awkward to handle because of the lack of so-called “body.” This inevitably necessitates that the polyimide precursor resin be so thickly applied and cured to a conductor as to form an ultimate polyimide layer of at least 20 microns thick. Since it is thus difficult to apply to a uniform thickness, thickness variations frequently occur, resulting in faulty products. This indicates the tendency that when the solution is applied in several divided portions, thickness variations become exaggerated with an increasing number of divided portions.
It was then proposed to form a thermoplastic polyimide layer on a conductor before lamination as disclosed, for example, in JP-A 1-244841 and JP-A 6-190967. With this method, the thermoplastic polyimide layer is pressure bonded so that the thickness of the entire polyimide layer becomes uniform. In particular, in the process of applying a polyimide or polyamide acid solution, drying and curing to form a thermoplastic polyimide/metal foil laminate, and bonding a polyimide film to the thermoplastic polyimide side under heat and pressure, as taught in JP-A 6-190967, the thermoplastic polyimide is melted by heating so that the thickness is corrected. As a result, the entire polyimide layer after laminated with the polyimide film has a uniform thickness.
Nevertheless, this process is not economical because the once cured polyimide must be bonded under heat and pressure, which requires a special equipment capable of heating to a temperature above the glass transition temperature (Tg) of polyimide.